1. Technical Field
The invention relates generally to gas turbine engine airfoils and particularly to gas turbine engine airfoils having increased fatigue strength.
2. Background Information
Modern gas turbine engines, particularly those used in aircraft, operate at high rotational speeds and high temperatures for increased performance and efficiency. There is a high demand for improved performance and efficiency because of the desire to increase the range an aircraft can fly without stopping to refuel.
Today's modern gas turbine engines rely primarily on nickel base and cobalt base superalloys for the material of the engine components in many critical applications, such as turbine vane and blade applications. As operating temperatures increase, however, the property limits of the base alloy materials are being approached.
Accordingly, attempts have been made to use coatings to protect certain components within the engine from the harsh operating environment. In particular, thermal barrier coating systems are increasingly employed to protect turbine blades, thereby extending the life of the blades and permitting enhanced fuel economy.
A typical thermal barrier coating system includes a combination of two different coatings, one being a metallic material (bond coat) located on the superalloy substrate and the other being a ceramic material (top coat) located on the metallic material. Exemplary ceramic materials are described in U.S. Pat. Nos. Re. 33,876 and 4,321,311.
The metallic material is present on the entire outer surface of the airfoil section and provides the substrate with resistance to oxidation degradation. The metallic material is often an aluminide or a MCrAlY material, where M is nickel, cobalt, iron, or mixtures thereof. Exemplary MCrAlY materials known in the gas turbine industry are the NiCoCrAlY coatings described by Hecht et al. in U.S. Pat. No. 3,928,026 and the NiCoCrAIY+Hf+Si coatings described by Gupta et al. in U.S. Pat. No. 4,585,481.
In addition to providing the substrate with resistance to oxidation degradation and corrosion, the metallic material serves to bond the ceramic material to the substrate. However, the metallic material provides added weight to the engine which may adversely affect engine efficiency.
Also, most conventional metallic bond coat materials tend to be less ductile than the base alloy material at normal engine operating temperatures and thus a fatigue debit may also result.
Accordingly, scientists and engineers working under the direction of Applicant's Assignee continue to address the need for gas turbine engine airfoils capable of withstanding the harsh operating environment. There is a particular need for airfoils having increased fatigue strength. This invention results from such effort.